Conventionally, inorganic photoconductive substances such as selenium, zinc oxide, cadmium sulfide, and silicon have widely been used in electrophotographic photoreceptors. These inorganic substances had many advantages and simultaneously had various disadvantages. For example, selenium has the disadvantages that its production conditions are difficult and it is liable to crystallize by heat or mechanical shock. Zinc oxide and cadmium sulfide have problems in moisture resistance and mechanical strength, and have the disadvantage such that electrostatic charging and exposure deterioration take place by a coloring matter added as a sensitizer, thus lacking in durability. Silicon involves that its production conditions are difficult, cost is expensive because of using a gas having strong irritating properties, and care should be taken to its handling because of being sensitive to humidity. Additionally, selenium and cadmium sulfide have toxicity problem.
Organic photoreceptors using various organic compounds that improve the disadvantages of the inorganic photoreceptors have widely been used. Organic photoreceptors include a single layer photoreceptor having a charge generating agent and a charge transport agent dispersed in a binder resin, and a multi-layered photoreceptor having a charge generating layer and a charge transport layer functionally separated. The characteristic of such a photoreceptor called a function-separated type is that a material suitable to the respective function can be selected from a wide range. Since a photoreceptor having an optional function can easily be produced, many investigations have been carried out.
As mentioned above, although various improvements such as development of new materials and combinations thereof have been conducted for the purpose of satisfying requirements such as fundamental performance and high durability required in electrophotographic photoreceptors, it is the present situation that sufficient organic materials are not yet obtained.
However, although organic materials have many advantages that are not possessed by inorganic materials, it is the present situation that organic materials sufficiently satisfying all of the properties required in electrophotographic photoreceptors are not obtained. That is, decrease in charge potential, increase in residual potential, change in sensitivity, and the like owing to repeated use give rise to deterioration of image quality. The causes of this deterioration are not completely clarified, but active gases such as ozone and NOX generated at charging by corona discharge, decomposition of a charge transport agent or the like by ultraviolet light and heat contained in light for exposure and light for removal of electricity, and the like are considered as some factors. For suppression of the deterioration, a method of combining a hydrazone compound and an antioxidant (for example, see Patent Document 1), a method of combining a butadiene compound and an antioxidant (for example, see Patent Document 2), and the like are known. However, organic materials having good initial sensitivity do not sufficiently improve deterioration in repeated use, and organic materials having less deterioration in repeated use have the problems in initial sensitivity and charging properties. Thus, it is the present situation that the effect for suppressing deterioration is not yet sufficiently obtained.    Patent Document 1: JP-A-1-044946    Patent Document 2: JP-A-1-118845